Jolliness is a characteristic of being happy and lighthearted. Santa Claus is said to express his jolliness with a cheerful "Ho ho ho!"
You may find your favorite teacher's jolliness to be contagious — he starts laughing and his jolliness spreads to the entire class. Jolliness, or the condition of being jolly, can be a temporary state (picture the audience at an incredibly funny movie), or an ingrained quality. Even if you're having a terrible day, your best friend's unwavering jolliness always cheers you up.